An integrated circuit (IC) is fabricated according to a layout design usable to form a plurality of masks for selectively forming or removing various layers of features, such as active regions, gate electrodes, various layers of isolation structures, and/or various layers of conductive structures. In some applications, an IC includes transistors having different threshold voltages. In one example, the transistors in the cells along a critical speed path of the IC having lower threshold voltages than those in the cells along a non-critical speed path of the IC. In another example, the gate structures at cell boundaries constitute dummy transistors and are adjusted to have higher threshold voltages than other functional transistors for reducing the current leakage through the dummy transistors.